2019 A Level H2 Chemistry Paper 1 Question 27 - Deduce Formula of Chromium Complex in Excess Hydroxide

Let Chemistry Guru, Singapore's renowned JC Chemistry tuition centre, guide you through 2019 A Level H2 Chemistry Paper 1 Question 27.

We are required to deduce the formula of the chromium containing species in the deep green solution.

The starting chromium complex in chromium (III) chloride should be [Cr(H₂O)₆]³⁺ where chromium (III) ion is surrounded by water ligands.

When hydroxide is added, the precipitate formed should be [Cr(H₂O)₃(OH)₃] due to ligand exchange between H₂O and OH^-.

When more OH^- is added, the remaining H₂O ligands will be substituted with OH^- to form [Cr(OH)₆]^3- complex which should give us the deep green solution.

Now we can go through the options.

A. [CrCl₄]^2-

This complex is not formed here but we can get this complex if concentrated HCl is added to provide a high concentration of Cl^- ligands.

B. [Cr(H₂O)₆]³⁺

This is our starting complex so is not the answer.

C. [Cr(OH)₆]^3-

This is the complex formed on adding excess OH^- ligands so this will be the answer to this question.

D. [Cr(OH)₃(H₂O)₃]

This is the initial precipitate formed hence is not the answer.

Topic: Transition Elements, Inorganic Chemistry, A Level Chemistry, Singapore

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